/**
 * configFile.ts
 * -------------
 * JSON config file loading + strict validation.
 *
 * Default path: ${XDG_CONFIG_HOME:-$HOME/.config}/move/config.json
 *
 * Schema (all keys optional; matching CLI flag names and units):
 *
 *   moveInterval   number   seconds, positive
 *   checkInterval  number   seconds, positive
 *   stepDelay      number   milliseconds, positive
 *   stepCount      number   pixels, positive
 *   verbose        boolean
 *
 * Unknown keys, wrong types, and non-positive numerics are rejected with a
 * `CliError` so the entry point can exit 2 (user error) with a clear
 * message pointing at the offending file.
 *
 * Return semantics:
 *   - `null` when no `explicitPath` was passed and the default path does
 *     not exist.  This is the "user has no config" happy path.
 *   - A `ConfigOverrides` when a file was found and validated.
 *   - Throws `CliError` if a problem is detected (missing explicit path,
 *     bad JSON, wrong shape, unknown keys, invalid values).
 */

import { existsSync, readFileSync, statSync } from "node:fs";

import { defaultConfigPath, type ConfigOverrides } from "./config.ts";
import { CliError } from "./errors.ts";

const ALLOWED_KEYS: ReadonlySet<string> = new Set<string>([
    "moveInterval",
    "checkInterval",
    "stepDelay",
    "stepCount",
    "verbose",
]);

function isPlainObject(value: unknown): value is Record<string, unknown> {
    return typeof value === "object" && value !== null && !Array.isArray(value);
}

function requirePositiveNumber(name: string, raw: unknown, path: string): number {
    if (typeof raw !== "number" || !Number.isFinite(raw) || raw <= 0) {
        throw new CliError(
            `invalid value for '${name}' in ${path}: ${JSON.stringify(raw)} (expected a positive number)`,
        );
    }
    return raw;
}

function requireBoolean(name: string, raw: unknown, path: string): boolean {
    if (typeof raw !== "boolean") {
        throw new CliError(
            `invalid value for '${name}' in ${path}: ${JSON.stringify(raw)} (expected a boolean)`,
        );
    }
    return raw;
}

/**
 * Load and validate the config file. See module docstring for return
 * semantics.
 *
 * @param explicitPath - If provided (e.g., from `--config`), the file
 *                       must exist and validate. If `undefined`, fall
 *                       back to `defaultConfigPath()`; a missing default
 *                       file is silent (returns `null`).
 */
export function loadConfigFile(explicitPath: string | undefined): ConfigOverrides | null {
    const required: boolean = explicitPath !== undefined;
    const path: string = explicitPath ?? defaultConfigPath();

    if (!existsSync(path)) {
        if (required) {
            throw new CliError(`config file not found: ${path}`);
        }
        return null;
    }

    if (!statSync(path).isFile()) {
        throw new CliError(`config path is not a regular file: ${path}`);
    }

    let raw: string;
    try {
        raw = readFileSync(path, "utf-8");
    } catch (err: unknown) {
        const msg: string = err instanceof Error ? err.message : String(err);
        throw new CliError(`could not read config file ${path}: ${msg}`);
    }

    let parsed: unknown;
    try {
        parsed = JSON.parse(raw);
    } catch (err: unknown) {
        const msg: string = err instanceof Error ? err.message : String(err);
        throw new CliError(`config file ${path} is not valid JSON: ${msg}`);
    }

    if (!isPlainObject(parsed)) {
        throw new CliError(`config file ${path} must contain a JSON object at the root`);
    }

    // Strict mode: reject any key we don't know about. Catches typos like
    // 'movInterval' that would otherwise sail through silently.
    for (const key of Object.keys(parsed)) {
        if (!ALLOWED_KEYS.has(key)) {
            const allowed: string = [...ALLOWED_KEYS].join(", ");
            throw new CliError(`unknown key '${key}' in ${path} (allowed: ${allowed})`);
        }
    }

    return {
        moveInterval:
            "moveInterval" in parsed
                ? requirePositiveNumber("moveInterval", parsed.moveInterval, path)
                : undefined,
        checkInterval:
            "checkInterval" in parsed
                ? requirePositiveNumber("checkInterval", parsed.checkInterval, path)
                : undefined,
        stepDelay:
            "stepDelay" in parsed
                ? requirePositiveNumber("stepDelay", parsed.stepDelay, path)
                : undefined,
        stepCount:
            "stepCount" in parsed
                ? requirePositiveNumber("stepCount", parsed.stepCount, path)
                : undefined,
        verbose:
            "verbose" in parsed
                ? requireBoolean("verbose", parsed.verbose, path)
                : undefined,
    };
}